1. Field of the Invention
The present invention relates to a single crystal silicon membrane, and more particularity to a single crystal silicon membrane with a suspension layer, a method for fabricating the same and a micro-heater.
2. Description of Related Art
Forming a suspension layer in a silicon substrate may use a heavily doped etching stop technique. With reference to U.S. Pat. No. 5,116,457, a heavily doped layer is formed on a silicon substrate, and multiple etching windows are formed on the heavily doped layer. First, the silicon substrate is immersed into an anisotropic etching solution. The etch rate of the undoped silicon substrate in the anisotropic etching solution is much faster than that of heavily doped silicon layer, and the etch selectivity of undoped silicon to heavily doped silicon is typically around 50. By this characteristic, the anisotropic etching solution will remove a non-heavily doped silicon substrate region through the etching windows to form a cavity. Because the etch rate of the undoped silicon substrate is much faster than that of heavily doped silicon layer, the doped layer could be preserved above the cavity to become the suspension layer. The ability to form the suspension layer is proportional to the etch selectivity of the silicon substrate to the suspension layer.
In general, a micro-heater is a suspension layer-based component which has high thermal isolation. A silicon micromachined micro-heater with a large heating area needs a big cavity, so a longer wet etching time is needed to form the big cavity. Even though the etch rate of the heavily doped silicon layer is slower than that of non-heavily doped region in an anisotropic etching solution, if the etching time is too long, the suspension layer will be over etched and the thickness of the suspension layer is difficult to control. One of the most critical technology of fabricating a silicon suspension layer is the enhancement of the etch selectivity of silicon substrate to suspension silicon layer.